The present invention relates to methods and apparatus for using optical fibers in curing, and particularly to the use of lossy fiber optics for optical and UV curing of resins and composite structures.
Composite materials consisting of fibers in a matrix of thermosetting polymer are well known and widely used in a variety of applications and industries, including the aircraft, automotive, spacecraft and marine industries. Typical fibers used in such composite materials include glass, carbon and polymer fibers, such as Kevlar. Typical matrix materials include polymers such as polyester or epoxy.
Traditionally the composite material in its raw state is pliable and readily manipulated to form a desired shape. Once in the desired shape, the material is cured, causing it to become rigid and maintain the desired shape even after removal of any molds or forms used to initially fashion the material into the desired shape. The curing may additionally cause bounding of the composite material to adjacent material.
Curing of the matrix of the composite material is caused by the addition of an energy source, which initiates a catalytic reaction. Energy sources include heat, light, and energetic electrons.
The most commonly used energy source for curing composite materials is heat. Thermally curing the resin has the advantage that high fiber fractions can be obtained, which provides high strength and low weight. However, thermoset resins have high tooling and manufacturing costs. The autoclaves necessary for curing thermosetting composites are expensive to purchase and operate. Tooling, such as the part molds, must be designed for these high temperatures and adequately compensate for a variety of thermal expansion issues. The high curing temperatures also lead to high residual stresses, which can be a particular problem for low temperature applications and composite/metal bonds. Part size is limited by the size of the autoclave and the cure time is very long (typically 10 hours or more).
Using energetic electrons is another established way to provide the energy necessary to cure composites. Electron beam (EB) curing minimizes the tooling costs, cure time, and residual stresses (if the parts are cured at their operating temperature), but typically sacrifices part performance when compared to traditional thermoset structures. Because an autoclave is not required, part size is not limited. However, EB curing may not provide uniform cures because the beam must be passed over the surface in a prescribed pattern. Lower fiber fractions are used with EB curing to allow electron penetration. Charge buildup within thick structures can also pose problems.
Light, particularly blue and Ultra-Violet (UV) wavelength light, is a third method of providing the energy for curing composite materials. Traditional light curing processes have similar advantages to EB curing relative to thermal curing. There are no size restrictions (no autoclave is required), tooling and manufacturing costs are reduced, cure time is greatly reduced, and curing can be performed over a wide temperature range.
Light curing of resins has been used extensively in situations where rapid curing is essential, such as dentistry, as exemplified by U.S. Pat. No. 6,435,872 to Nagel entitled xe2x80x9cTapered light probe with non-circular output for a dental light curing unitxe2x80x9d, the contents of which are hereby incorporated by reference, and U.S. Pat. No. 6,200,134 to Kovac et al. entitled xe2x80x9cApparatus and method for curing materials with radiationxe2x80x9d, the contents of which are hereby incorporated by reference. The systems described by Nagel and by Kovac et al are both conventional in that the light is transmitted along the fiber and exits from the fiber end external to the curing resin.
However, traditional blue light or UV curing has significant drawbacks. It is limited to thin layups with transparent fibers, e.g., no carbon fiber. Layups also have lower fiber fraction to allow light or UV penetration, which also minimizes performance. Traditional UV curing using external illumination cannot cure thick sections, except by curing multiple thin layups.
The main difficulty with light curing of composite materials has been supplying the energy source from outside the matrix to the interior in a uniform manner with sufficient flux to affect a cure in a short time.
The light curing method and apparatus of this invention overcomes these disadvantages while maintaining all of the advantages of light curing by delivering the light to the photocurable resin in a composite material in a unique wayxe2x80x94by means of one or more lossy optical fibers embedded in, or in close proximity to, the light-curable resin.
Although lossy optical fibers have been used for illumination and decoration as described in, for instance, by Baker in U.S. Pat. No. 5,502,903, entitled xe2x80x9cFootwear with illuminated linear opticsxe2x80x9d, the contents of which are hereby incorporated by reference, the linear optics described previously are largely limited to having relatively thick (of the order of 0.165 inch diameter) polymer cores with thin air gaps formed by heat shrinking material over the core, as described for instance by Robbins et al. in U.S. Pat. No. 5,221,387 entitled xe2x80x9cMethods of manufacture of improved linear optical conduitsxe2x80x9d, the contents of which are hereby incorporated by reference. The previously described materials and methods used to make the fibers lossy are not generally suitable for use throughout the optical spectrum and are particularly unsuitable for short wavelength radiation, including UV radiation.
The invention of this application is a light curing method and apparatus that utilizes one or more lossy optical fibers to deliver light to a curable matrix material. In particular, this invention is a method and apparatus in which the light delivered to the curable resin is the light leaked out of the light transmitting and leaking fiber.
Light curable resins are often activated only or more efficiently by shorter wavelengths of light, such as, but not limited to those wavelengths conventionally described as either blue or UV light and much of the description of the invention is in terms specific to blue or UV light. However, one of skill in the art will readily appreciate that the invention is operable with any wavelength of electromagnetic radiation for which there is both a radiation curable resin and fibers capable of transmitting and leaking that wavelength of electromagnetic radiation.
In one embodiment of the invention, an array of lossy optical fibers is interleaved with structural fibers in the composite. Because the light source is effectively embedded in the structure, the resin is cured uniformly, regardless of the reinforcing fiber properties, structure size, shape, or thickness.
The light transmitting and leaking fibers may be made lossy for the purposes of this invention by means such as, but not limited to, having a cladding sufficiently thin to allow leakage by evanescent wave or by bends in the fiber, including micro-bends introduced by weaving the fiber.
In another embodiment of the invention, the light transmitting and leaking fibers may also form part of the structure.
The method of this invention provides several advantages over existing methods of curing composite materials including, but not limited to, faster curing times, the ability to include materials with limited light transmission, no size limits imposed by the need for an autoclave, the ability to cure materials at their operational temperature and so minimize thermal stresses, and no thickness limitations on the composite material.
As photo-curing materials tend to work better with shorter wavelength light, in another embodiment of this invention, the lossy fibers are made capable of transmitting and leaking short wavelength radiation such as blue light or Ultra-Violet (UV) radiation.